June 11, 2023

The toupper() function is a standard library function in the C programming language. It is defined in the ctype.h header file and is used to convert lowercase letters to uppercase letters in a string.

The basic syntax of the toupper() function is:

int toupper(int c);

Here, the parameter c is an integer that represents the character to be converted. The function returns the uppercase version of the input character if it is a lowercase letter. If the input character is already uppercase or not a lowercase letter, the function simply returns the input character.

The toupper() function is often used in applications that involve string manipulation, such as text processing, data cleaning, and data validation. It is also useful for case-insensitive searches and comparisons.

Examples

Let’s take a look at a few examples of how the toupper() function can be used:

Example 1: Converting a string to uppercase

In this example, we use the toupper() function to convert a string to uppercase. We create a string and then iterate through each character in the string, converting each lowercase letter to uppercase using the toupper() function.

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#include <ctype.h>
#include <stdio.h>

int main() {
    char str[] = "this is a test string";
    int i;

    for (i = 0; str[i] != '\0'; i++) {
        str[i] = toupper(str[i]);
    }

    printf("%s\n", str);

    return 0;
}

Output:

1

THIS IS A TEST STRING

Example 2: Comparing strings without regard to case

In this example, we use the toupper() function to compare two strings without regard to case. We first convert both strings to uppercase using the toupper() function and then use the strcmp() function to compare the uppercase strings.

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#include <ctype.h>
#include <stdio.h>
#include <string.h>

int main() {
    char str1[] = "This is a test string";
    char str2[] = "THIS IS A TEST STRING";
    int i;

    for (i = 0; str1[i] != '\0'; i++) {
        str1[i] = toupper(str1[i]);
    }

    for (i = 0; str2[i] != '\0'; i++) {
        str2[i] = toupper(str2[i]);
    }

    if (strcmp(str1, str2) == 0) {
        printf("The two strings are equal.\n");
    } else {
        printf("The two strings are not equal.\n");
    }

    return 0;
}

Output:

1

The two strings are equal.

The toupper() function is a simple yet powerful tool that is essential for many applications that involve string manipulation. It is important to note that the toupper() function only converts lowercase letters to uppercase and does not affect other characters. Additionally, the function does not modify the input character if it is already uppercase or not a lowercase letter.

Possible implementation

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#include <ctype.h> // islower

int toupper(int chr)
{
    if (islower(chr)) {
        return (chr - ('a' - 'A'));
    } else {
        return chr;
    }
}

➡️ The importance of clean code in C and Python programming

⬅️ libc - tolower()

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